We propose to: (1) address fundamental questions about social networks in relation to health and health-related behaviors; and (2) develop innovative methodologies and technologies to facilitate, improve, and expand the capabilities of SNA by employing innovative network analytic methods and state-of-the-art modeling approaches to better understand school-based peer network effects on substance use behaviors from 6th-12th grade. Using an ecological framework, we will develop models linking AOD use to peer networks and other important features of the social context. School-level analyses, dyadic analyses, trajectory analyses and longitudinal co-evolutionary models for networks and behavior will examine how school-based peer network effects are related to substance use, allowing us to determine which effects are common across those behaviors. Integrating network data into models that facilitate an ecological perspective and modeling network effects alongside other known psychosocial risk factors will help us understand how peers influence substance use behaviors more fully than ever before. The proposed study is the first to compare complete and ego-centered study designs to assess whether and how network data collection strategies introduce error and affect analyses relating observed social behaviors (e.g., friendship) to aspects of health (e.g., AOD use). As such, it promises to significantly improve our understanding of these designs, facilitating their use and expanding their application. We have the unique opportunity to investigate these issues in the context of on an ongoing longitudinal study of peer relationships managed by the University of Illinois. A distinctive feature of this dataset is that complete peer network information (grade-based friendship networks) and self-reported AOD use have been collected at each of the existing waves for some 1232 adolescents. We will continue following this cohort through 12th grade (two additional waves of data collection) to examine the impact of peer networks on AOD use during the entire period from 6th-12th grade. We will also collect ego-centered (i.e., personal) network data from all students in one of the three schools to assess the impact of network-based study designs on understanding the link between peer networks and adolescent AOD use. This study will generate an unprecedented data set for analyses that will advance our knowledge of how peer networks shape AOD use in youth, and contribute much-needed information on the application of novel SNA methods in public health research.